The present invention relates to the preparation of isocyanates.
Organic isocyanates have many known uses and there are several known methods for their synthesis. Many methods are based on the knowledge that phosgene will react with active hydrogen-containing compounds. For example, U.S. Pat. No. 4,123,450 teaches the reaction of phosgene with phenol in the presence of NaOH in a two-phase reaction system to make phenyl chloroformate, which in turn is reacted with an alkylamine in a two-phase system to give the corresponding N-alkylcarbamate, which can be pyrolyzed to yield the alkyl isocyanate. U.S. Pat. No. 4,128,569 teaches the manufacture of isocyanates by reacting phosgene and a primary amine, including aromatic diamines having the divalent bridging groups --O--, --SO.sub.2 --, --CH.sub.2 --, and --C(CH.sub.3).sub.2 --. U.S. Pat. No. 3,440,268 teaches the production of isocyanates by reacting a primary amine with phosgene to form the corresponding carbamoyl chloride, and then heating the carbamoyl chloride to form the corresponding isocyanate plus hydrogen chloride. These methods do not employ an aminophenol in the preparation of isocyanates.
More specifically, organic isocyanates having the isocyanato moiety bonded to a tertiary alkyl carbon atom are especially useful, and several methods for their preparation are known. U.S. Pat. No. 3,290,350 teaches the preparation of tetramethylxylylene diisocyanates (hereinafter TMXDI's) by reacting isocyanic acid and certain vinylidene compounds, including aromatic vinylidene compounds. U.S. Pat. No. 4,377,530 teaches an improvement to the method of U.S. Pat. No. 3,290,350, using a high excess of the isocyanic acid. U.S. Pat. No. 4,361,518 teaches the production of TMXDI's by reacting the corresponding halides with an excess of isocyanic acid. Similarly, U.S. Pat. No. 4,379,767 teaches a two-step method for the preparation of TMXDI's. The method involves reacting an aromatic diisopropenyl compound with a carbamoyl halide to form the tertiary benzyl halide, which in turn is reacted with isocyanic acid to form the corresponding TMXDI. U.S. Pat. No. 4,395,369 teaches a similar method, and teaches that the aromatic moiety can contain --O--, --S--, --CO-- and polymethylene linkages. U.S. Pat. Nos. 4,399,073 and 4,399,074 teach the production of TMXDI's by reacting alkali metal cyanates and t-alkylhalides under very specific reaction conditions. Thus, these known methods for producing TMXDI's require the use of cyanate salts or isocyanic acid and do not employ aminophenols. Isocyanic acid is hard to handle in that it tends to trimerize to form isocyanurates. Additionally, it is a severe explosion hazard. The reaction of organic halides with cyanate salts does not proceed readily except under very specific reaction conditions and with a limited number of catalysts.
U.S. Pat. No. 4,439,616 teaches a multistep process for the preparation of TMXDI's. The process involves preparing a carbamate from an olefin and, e.g., a carbamic acid alkyl ester in the presence of an acid catalyst. The acid catalyst is then completely neutralized, and the carbamate is then thermally cracked to form the corresponding TMXDI. Said method does not employ an aminophenol.
U.S. Pat. No. 3,488,376 teaches the phosgenation of a m- or p-aminophenol in an aprotic, polar solvent to give the corresponding m- or p-isocyanatophenol. In said process, only the amino moiety reacts with the phosgene.
In view of the deficiencies of the prior art methods, it would be desirable to provide a simple, one-step process for the preparation of aromatic alkyl diisocyanates, including especially useful novel aromatic tertiary alkyl polyisocyanates, which method would not require the use of cyanate moiety-containing reactants.